In addition, it’s usually not possible to just simply color a black and white drawing because they where never ment to be in collor, sooner or later you run into problems. For example the blood in the picture is stylized by crosshatching. If you just put some red there without editing the hatching away it would look weird. Same for the solid black shadows on the clothes. Painting over it with just color or any other mixing blending mode will just leave it black. If they made heavy use of screentones that will look strange too.
From my experience it worked best for me to ask the artist if the lineart is still available (just outlines without any filling) or if that’s not possible or the lineart not available anymore, editing the black and white drawing so it can be easily colored which either means tracing the line art or erasing everything inside. This sounds like a lot of extra work but it gives the best results. With the drawing properly prepared, coloring it is just a matter of minutes with the colorize mask tool.